There is an increasing demand for reduction in power consumption (high efficiency) in transmission power amplifiers for cellular phone base stations. As a matter of course, the reduction in power consumption is effective not only in reducing an environmental load by saving electricity costs or energy, but also in reducing the surface area of a radiator plate necessary for heat radiation, because of a reduction in calorific power of power amplifiers. This results in an effect of allowing the volume of each transmission power amplifier to be made relatively small.
To improve the efficiency of transmission power amplifiers, Doherty amplifiers are typically used (see Patent Literature 1, for example). A Doherty amplifier includes a carrier amplifier that performs a signal amplification operation constantly, and a peak amplifier that operates only at the time of high power output. The Doherty amplifier has a configuration in which an input signal is distributed to the carrier amplifier and the peak amplifier, and outputs of the carrier amplifier and the peak amplifier are combined.
FIG. 6 is a block diagram showing an exemplary power amplifier with a Doherty amplifier configuration. Referring to FIG. 6, the power amplifier includes an input branch circuit 6, DC (Direct Current) decoupling capacitors 8, a carrier amplifier 2, and a peak amplifier 3.
The power amplifier also includes RF (Radio Frequency) choke coils 9, a DC power supply 1, an output combiner circuit 7, an RF input terminal 21, and an RF output terminal 22. Further, the input branch circuit 6 includes a ¼ wavelength transmission line 4, and the output combiner circuit 7 includes a ¼ wavelength transmission line 5.
As the carrier amplifier 2 and the peak amplifier 3, field effect transistors (FET) are used, for example. The output combiner circuit 7, which is formed of a transformer, is typically composed of the ¼ wavelength transmission line 5. The input branch circuit 6 is composed of the ¼ wavelength transmission line 4 or a 90-degree hybrid circuit, for example, which causes output signals of the carrier amplifier 2 and the peak amplifier 3 to be in phase at a signal combining point of the output combiner circuit 7.
A common power supply voltage from the DC power supply 1 is supplied to each of the carrier amplifier 2 and the peak amplifier 3 through the RF choke coil 9. As the DC decoupling capacitors 8, capacitances having a sufficiently low impedance at the frequency of an RF signal to be used are selected. In general, the carrier amplifier 2 is based to AB-class or B-class, and the peak amplifier 3 is biased to C-class. The power amplifier includes the carrier amplifier 2 that operates while being saturated in the vicinity of a saturated output power. Accordingly, an efficiency higher than that of a normal A-class or AB-class amplifier is realized even when the saturated output power backs off.
FIG. 7 is a graph showing an example of efficiency versus output signal power characteristics of the power amplifier with a Doherty amplifier configuration shown in FIG. 6. FIG. 7 shows that when the saturated output levels of the carrier amplifier and the peak amplifier are the same, the power amplifier has efficiency peaks at a 6 dB backoff point where the carrier amplifier is saturated and at a 0 dB backoff point where the peak amplifier is also saturated, with respect to the combined saturated outputs of the carrier amplifier and the peak amplifier. FIG. 8 is a signal power versus time characteristic diagram showing a relationship between a peak power value and an average power value of the RF signal.
Meanwhile, W-CDMA modulation wave and OFDMA modulation wave, which are used in recent mobile communication systems, have a relatively large peak factor (a ratio between an average power level and a peak power level of an input signal) of 7 dB to 11 dB. For this reason, it is necessary to set the operating point of each power amplifier in the range of 7 dB to 11 dB or higher. Accordingly, it is impossible to cause the power amplifier to operate at the efficiency peak point.
An example of means for solving such a problem is disclosed in Patent Literature 2. This supplies different power supply voltages to a carrier amplifier and a peak amplifier and changes the saturated output levels of the carrier amplifier and the peak amplifier, thereby allowing the efficiency peak point of a Doherty amplifier to change from 6 dB and to operate at a given operating point with a maximum efficiency. Further, Patent Literature 2 proposes a technique in which an input level detector and a voltage control unit are provided. When the input level is relatively low, a power supply voltage is set to be relatively low, and when the input level is relatively high, the power supply voltage is set to be relatively high, thereby optimizing the power supply voltage supplied to each of the carrier amplifier and the peak amplifier according to the input average signal power and maintaining the high efficiency.
Another example of the related Doherty amplifier is disclosed in Patent Literature 3. This amplifier has a configuration in which an input signal is branched by a directional coupler; the branched signals are further distributed by a distributor; an envelope of one of the distributed signals is detected by an envelope detector; and a bias voltage according to a peak/average power ratio is applied to a peak amplifier.
Still another example is disclosed in Patent Literature 4. In the technique disclosed in Patent Literature 4, input distribution means distributes an input signal and inputs one of the distributed signals to a detection circuit. Next, the detection circuit detects an envelope of the input signal and inputs the detected output to waveform forming means. Then, the waveform forming means outputs a signal corresponding to the envelope of an equivalent RF signal to each of voltage controllers 1 and 2. After that, the voltage controller 1 supplies a voltage corresponding to the signal from the waveform forming means to a carrier amplifier, and the voltage controller 2 supplies a voltage corresponding to the signal from the waveform forming means to a peak amplifier.